Calcium stearate is a multipurpose polymer additive and is widely used in the polymer industry as a lubricant, processing aid and stabilizer. The single biggest use is in conjunction with primary organotin mercaptide heat stabilizers in the extrusion of rigid PVC pipe, siding and profiles. Other significant uses are in polyolefins, phenolics, reinforced polyester and rubber.
Calcium stearate is as an internal lubricant and secondary heat stabilizer used in rigid PVC extrusion applications such as pipe, siding and profile where it is generally used at levels between 0.5 and 2.0 parts per hundred parts resin. Calcium stearate is thought to be effective at these low use levels because of a combination of properties including its polarity; its waxy character; its dispensability; its limited compatibility; and its melt viscosity at polymer processing temperatures. The effectiveness of calcium stearate is somewhat limited by its instability in the presence of strong acids, its tendency to plate out and its propensity to build up static charge and to form combustible dusts in air.
Without being held to any one theory of operation or mechanism, it is widely believed that calcium stearate functions as a secondary heat stabilizer in rigid PVC by reacting with and absorbing HCl generated as a result of heat degradation of the polymer during processing as illustrated in Equation 1.(RCOO)2Ca+2HCl→CaCl2+2RCOOH  (1)When calcium stearate is consumed in this manner its effectiveness as an internal lubricant is diminished.
Calcium stearate is supplied commercially as either a fine (sub 325 mesh) powder or a free flowing (20-100 mesh) granule. The granular product is the preferred form because of dust explosivity and plant hygienic issues associated with the handling of fine powders. Calcium stearate is usually formed by the reaction of stearic acid with calcium hydroxide (slaked lime) or calcium oxide (quick lime).
Calcium hydroxide and calcium oxide also find limited use as polymer additives. Calcium hydroxide is used as an acid neutralizer and calcium oxide is used primarily as a desiccant. Both materials are non-melting at polymer processing temperatures and as a result must be ground to extremely fine powders before the incorporation into polymer compounds. Both materials are highly polar and as a result are difficult to disperse in non-polar polymers with high melt viscosities. Calcium hydroxide and calcium oxide powders have a strong tendency to absorb CO2 and moisture which give rise to poor handling properties, passivated surfaces and loss of effectiveness. Both present significant hygienic and plant housekeeping problems to the compounder.
A combination of economic and environmental issues have prompted rigid PVC formulations to continually strive to lower the use levels or completely replace primary heat stabilizers based upon organotin, barium, cadmium, and lead as well as compounding ingredients which are difficult to handle.
Both calcium stearate and calcium hydroxide function as secondary heat stabilizers for rigid PVC by absorbing HCl. There is a synergistic effect when calcium stearate and calcium hydroxide are used in combination. From a practical standpoint, however, the addition of both calcium stearate and calcium hydroxide separately represents an additional compounding step exacerbated by the additional handling problems associated with fine ground calcium hydroxide. Adding a single physical pre-blend of calcium stearate and calcium hydroxide also raises problems in that granules are the preferred form for calcium stearate because of dust explosivity concerns and fine ground powder is the required form for effectiveness of calcium hydroxide. Furthermore, the significant differences in particle size between granular calcium stearate (20 mesh) and fine ground calcium hydroxide (sub 325 mesh) coupled with the differences in specific gravity between calcium stearate (1.0) and calcium hydroxide (2.3) preclude the preparation of homogeneous non-segregating, storage stable blends of calcium stearate and calcium hydroxide. Melt blending of calcium stearate with any significant quantity of Ca(OH)2 and subsequent grinding is not an option because calcium stearate decomposes at the melting point of Ca(OH)2 and the melt viscosity of calcium stearate is too viscous to allow for the effective dispersion therein of any significant quantity of fine ground Ca(OH)2. Polymer bound concentrates of calcium stearate and calcium hydroxide are not practical because of cost considerations.
It would therefore be desirable to have a heat stabilizer exhibiting desirable aspects of both calcium stearate and calcium hydroxide without the attendant physical handling and blending issues.